DESCRIPTION: While hormone replacement therapy greatly reduces the risk of cardiovascular disease in postmenopausal women, the mechanisms underlying the reduction in risk are only partly understood. This study examines the effects of hormone replacement therapy on adrenergic physiology which plays a major role in cardiovascular regulation. The investigators will study 75 postmenopausal women randomly divided into three groups, an estrogen replacement therapy group, an estrogen combined with progesterone replacement therapy group, and a placebo group. All subjects will be studied twice, prior to and after a 12 week treatment period. Lymphocyte beta (2)-and platelet alpha (a)-adrenergic receptors and plasma catecholamines, and blood pressure, heart rate and hemodynamic responses at rest and in response to behavioral stress will be determined. Blood pressure and heart rate will also be determined in the ambulatory setting. Pressor sensitivity and barroreflex sensitivity will be determined via phenylephrine infusion, beta-adrenergic receptor sensitivity will be determined by isoproterenol infusion, and norepinephrine release and clearance rates will be determined via [3H]-norepinephrine infusion. The project will help elucidate the complex effects of estrogen and progesterone therapy on adrenergic regulation of the cardiovascular system in postmenopausal women.